Apparatus and circuit for power supply, and apparatus for controlling large current load Apparatus and circuit for power supply,and apparatus for controlling large current load

ABSTRACT

A GaN-FET ( 10 ) having a small resistance at the time of conduction is disposed in the path of a main current that flows between an input terminal ( 11 ) and an output terminal ( 12 ) . Moreover, there are provided a Zener diode (ZD) and a resistor (R) for setting an output voltage (Vout). As a result, reduction of size and weight is realized, and design including radiation design can be conducted flexibly. Time and labor required for design are remarkably reduced.

TECHNICAL FIELD

[0001] The present invention relates to various power supply apparatusesused in automobiles, electric vehicles, construction machinery, variouspublic welfare devices (such as video devices, television sets, andaudio devices), various industrial devices (such as personal computers,communication devices, and FA control devices), and so on. Furthermore,the present invention relates to a power supply circuit and a largecurrent load control apparatus of switching power supply using GaN-FETs.

BACKGROUND ART

[0002] Heretofore, power semiconductor devices such as diodes,thyristors, triacs, GTO (Gate Turn Off) thyristors, bipolar transistors,MOS-FETs, and IGBTs (Insulated Gate Bipolar Transistors) are used invarious power supply apparatuses. In these power semiconductor devices,a main current that flows through the power semiconductor device iscontrolled by switching control or analog control. These powersemiconductor devices are devices serving as nuclei for implementingstabilized power supply apparatuses, such as switching regulators andlinear regulators, and inverters for performing conversion to powerhaving an arbitrary frequency and an arbitrary output voltage.

[0003] In these power semiconductor devices, there are switching losscaused by superposition of transient voltage and current at the time ofswitching and conduction loss caused at the time of conduction. Theselosses are converted mainly to heat. By the way, the conduction loss hassuch a characteristic as to become small as the on-resistance isdecreased. This on-resistance corresponds to composite resistance of thechannel resistance, bulk resistance, and so on existing within thesemiconductor except contact resistance between electrodes andsemiconductor layer interfaces in the power semiconductor device. Heatgenerated by the power semiconductor device causes a temperature rise ofthe power semiconductor device itself. Due to this temperature rise, thepower semiconductor device operates at high temperature. Due to thishigh temperature operation, heat generation of the power semiconductordevice is promoted. Such positive feedback is caused. As a result,thermal destruction of the power semiconductor device is caused bythermal runaway.

[0004] Usually in the power supply apparatus, therefore, the powersemiconductor device itself is provided with a radiation mechanism andin addition, with a radiator such as radiation fins for radiating heatgenerated by the power semiconductor device. Furthermore, a radiationfan is provided in order to improve the radiation effect in some cases.Furthermore, there is provided a fail safe mechanism that senses thetemperature of the power semiconductor device and stops operation of thepower semiconductor device when the temperature rises to such a value asto cause thermal runaway.

[0005] However, the radiator is formed of a good thermal conductivematerial, such as aluminum, in order to provide the radiator with afunction of heat sink as well. This results in a problem that the wholepower supply apparatus becomes large in weight and capacity. Especiallyas for mobile power supply apparatuses for vehicles or portable powersupply apparatuses, emergence of power supply apparatuses reduced insize and weight is demanded strongly.

[0006] For example, in a conventional power supply apparatus shown inFIG. 16, a radiator 302 having a large weight and a large capacity isneeded. The power supply apparatus shown in FIG. 16 is a DC-DC converterpower supply apparatus of a vehicle. A MOS-FET using a Si semiconductormaterial is incorporated in the power supply apparatus as a switchingelement. The apparatus main body 301 encloses all the elements formingthe power supply apparatus. On the top of the apparatus main body 301, aradiator 302 formed of aluminum is provided. On a junction interfacebetween the radiator 302 and the apparatus main body 301, a MOS-FET,which is not illustrated, stick to the radiator 302. Heat generated bythe MOS-FET is absorbed by the radiator 302, and radiated by finsdisposed on the top of the radiator 302. Because of installation of theradiator 302, the weight and volume of the whole power supply apparatusbecome excessively large.

[0007] Furthermore, the radiator needs to stick to the powersemiconductor device in order to favorably transfer the heat from thepower semiconductor device. This brings about limitation on design thatthe radiator needs to be disposed with due regard to the outer peripheryof the casing of the power supply apparatus and the radiation path. Thisresults in a problem that the degree of freedom of the power supplyapparatus is reduced. In addition, as for devices such as vehicles usingthe power supply apparatus, design of the whole device must be changedaccording to the disposition position of the power supply apparatus.Thus, there is also a problem that the design of the whole device islargely affected.

[0008] In addition, in designing the radiator, it is necessary toconduct sufficient radiation design with due regard to the ambientenvironment of the power supply apparatus. In addition, it is necessaryto prevent the power semiconductor device, which is a heat source, fromaffecting other circuit elements having low heat-resisting property.This results in a problem that much time and labor are required forradiation design and arrangement design of other circuit elementsincluded in the power supply apparatus.

[0009] Furthermore, a heat protection circuit for preventing thermalrunaway of the power supply apparatus is needed. This heat protectioncircuit monitors the temperature changes of important components, suchas the power semiconductor device, included in the power supplyapparatus. When the temperature has risen to a predetermined value, theheat protection circuit conducts fail safe control, such as stopping thepower supply apparatus and causing shift to a low dissipation mode. Thisheat protection circuit is a complicated circuit that senses thetemperature, output current, and so on and conducts a shift to fail safecontrol by using a logic processing circuit. Thus, there is a problemthat the power supply apparatus must have a heat protection circuithaving such a complicated circuit.

[0010] Recently as semiconductor devices having high heat-resistingproperty, high breakdown voltage, high operation rate and low conductionloss, GaN (galliumnitride) FETs (Field Effect Transistors) have beendeveloped.

[0011] Heretofore, such power supply circuits have been applied to, forexample, automobiles, various public welfare devices (such as video,television and audio devices), and industrial devices (such as personalcomputers, communication devices, and FA control devices).

[0012] The above described power supply circuit includes a transformer.A transistor made of, for example, a power MOS element turns on and offaccording to a gate signal. As a result, an output voltage is generatedon a secondary winding side.

[0013] In the above described power supply circuit, however, the powerMOS element used as the transistor, such as a power MOS-FET (2SK2313)generates much heat. Therefore, it is necessary to perform the radiationdesign accurately. A channel temperature Tch max of the power MOS-FETitself at an ambient temperature of 85° C. is calculated as$\begin{matrix}{{{Tch}\quad \max} = {{{Ta}\quad \max} + {{Ptotal} \times {{Rth}\left( {{ch} - a} \right)}}}} \\{{= {85{^\circ}\quad {C.{+ 2}}W \times 50{^\circ}\quad {{C.}/W}}}\quad} \\{= {185{^\circ}\quad {C.}}}\end{matrix}$

[0014] where Ta max: ambient temperature

[0015] Ptotal: total loss

[0016] Rth (ch-a): thermal resistance between channel and environment.

[0017] The temperature rises up to the channel temperature or higher.Therefore, it is necessary to provide a radiation plate. Supposingderating of 50° C. for a channel temperature of 150° C., the radiationplate design is represented asθ  f < θ  ch − a − (θ  I + (θ  c + θ  s)) ${\begin{matrix}{= {{7.5{^\circ}\quad {{C.}/W}} -}} \\{= {5.9{^\circ}\quad {{C.}/W}}}\end{matrix}\left( {{0.833{^\circ}\quad {{C.}/W}} + \quad {0.8{^\circ}\quad {{C.}/W}}} \right)}\quad$

[0018] where θf: thermal resistance of radiator

[0019] θch-a: total thermal resistance between channel and environment

[0020] θi: thermal resistance between junction portion and case(internal thermal resistance)

[0021] θc+θs: thermal resistance between case and radiator From theforegoing description, it is necessary to select a radiator having athermal resistance of 5.9° C./W or less. For example, therefore, aradiation plate made of an aluminum plate of 100 cm² having a thicknessof 1 mm becomes necessary. As a result, the conventional power supplycircuit has a problem that the circuit configuration becomes large andheavy because of the radiation plate.

[0022] Furthermore, heretofore, such a large current load controlapparatus is applied to, for example, lighting control of head lamps ofautomobiles.

[0023] In the above described large current load control apparatus,lighting control of a head lamp is conducted by turning on and off apower MOS-FET formed of, for example, an on/off control switchingelement provided on a power supply line, which connects a battery to thelamp, under the control of a microcomputer.

[0024] In this control apparatus, however, the power MOS-FET used as theswitching element of on/off control generates much heat. Therefore, itis necessary to perform the radiation design accurately. A channeltemperature Tch max of the power MOS-FET is calculated as$\begin{matrix}\begin{matrix}{{{Tch}\quad \max} = {\left( {{Ta}\quad \max} \right) + {\left( {{Ron}\quad \max} \right) \times \left( {{lo}\quad \max} \right) \times \left( {{lo}\quad \max} \right) \times {{Rth}\left( {{ch} - a} \right)}}}} \\{= {85{^\circ}\quad {C.{+ 0.013}}\Omega \times 10A \times 10A \times 50{^\circ}\quad {{C.}/W}}} \\{= {150{^\circ}\quad {C.}}}\end{matrix} & (10)\end{matrix}$

[0025] where Ta max: ambient temperature

[0026] Ron max: on-resistance

[0027] lo max: current value

[0028] Rth (ch-a): thermal resistance between channel and environment.

[0029] The temperature rises up to the channel temperature. Therefore,it is necessary to provide a radiation plate. Supposing derating of 50°C. for a channel temperature of 150° C., the radiation plate design isrepresented as θ  f < θ  j − a − (θ  I + (θ  c + θ  s))$\begin{matrix}{= {{11.5{^\circ}\quad {{C.}/W}} - \left( {{0.833{^\circ}\quad {{C.}/W}} + {0.8{^\circ}\quad {{C.}/W}}} \right)}} \\{= {9.9{^\circ}\quad {{C.}/W}}}\end{matrix}$

[0030] where θf: thermal resistance of radiator

[0031] θj-a: total thermal resistance between channel junction portionand the outside air

[0032] θi: thermal resistance between junction portion and case(internal thermal resistance)

[0033] θc+θs: thermal resistance between case and radiator From theforegoing description, it is necessary to select a radiator having athermal resistance of 9.9° C./W or less. For example, therefore, aradiation plate made of an aluminum plate of 6 cm² having a thickness of1 mm and a weight of approximately 10 g becomes necessary. As a result,the conventional large load control apparatus has a problem that thecircuit configuration becomes large and heavy because of the radiationplate.

[0034] Therefore, it is one object of the present invention is toprovide a power supply apparatus capable of implementing reduction ofthe size and weight, conducting flexibly design including the radiationdesign, and remarkably reducing the time and labor required for thedesign.

[0035] Furthermore, it is an another object of the present invention isto provide a power supply circuit capable of reducing the heat generatedby the transistor, thereby making the radiation plate unnecessary, andimplementing reduction of the size and weight of the circuit.

[0036] Furthermore, it is still another object of the present inventionis to provide a large current load control apparatus capable of reducingthe heat generated by the on/off control switching element, therebymaking the radiation plate unnecessary, and implementing reduction ofthe size and weight of the circuit.

DISCLOSURE OF THE INVENTION

[0037] A power supply apparatus according to the present inventioncomprises: a semiconductor element disposed in the path of a maincurrent that is a subject of power control and formed by using a GaNcompound; and control unit which controls conduction of the main currentflowing through the semiconductor element.

[0038] According to the above-mentioned aspect of the present invention,a semiconductor element formed by using a GaN compound is disposed inthe path of a main current that is a subject of power control, andcontrol unit controls conduction of the main current flowing through thesemiconductor element. Since the semiconductor element is small inresistance at the time of conduction, little heat is generated and itbecomes unnecessary to provide the power supply apparatus with aradiator. Furthermore, it becomes unnecessary to make the semiconductorelement stick to a radiator. The semiconductor element can be disposedin an arbitrary position in the power supply apparatus.

[0039] Furthermore, a power supply apparatus according to the presentinvention comprises: a semiconductor element disposed in the path of amain current that is a subject of power control and formed by using aGaN compound; and control unit which conducts switching control onconduction of the main current flowing through the semiconductorelement.

[0040] According to the above-mentioned aspect of the present invention,a semiconductor element formed by using a GaN compound is disposed inthe path of a main current that is a subject of power control, andcontrol unit conducts switching control on conduction of the maincurrent flowing through the semiconductor element. Since thesemiconductor element is small in resistance at the time of conduction,little heat is generated and it becomes unnecessary to provide the powersupply apparatus with a radiator. Furthermore, it becomes unnecessary tomake the semiconductor element stick to a radiator. The semiconductorelement can be disposed in an arbitrary position in the power supplyapparatus.

[0041] Furthermore, a power supply apparatus according to the presentinvention comprises: a plurality of the semiconductor elements, and theplurality of the semiconductor elements are connected in parallel, inthe above described invention.

[0042] According to the above-mentioned aspect of the present invention,the power supply apparatus includes a plurality of the semiconductorelements disposed in the path of a main current that is a subject ofpower control, and each of the semiconductor elements are connected inparallel. The limit of the main current that can be controlled isremarkably improved. Even if the semiconductor elements are connected inparallel, the semiconductor elements generate little heat, thetemperature of the semiconductor elements themselves rises little. Acurrent unbalance among the semiconductor elements caused by dispersionof the temperature characteristic is slight.

[0043] Furthermore, a power supply apparatus according to the presentinvention, wherein the plurality of semiconductor elements are arrangedso as to be adjacent to each other, in the above described invention.

[0044] According to the above-mentioned aspect of the present invention,the power supply apparatus includes a plurality of the semiconductorelements disposed in the path of a main current that is a subject ofpower control, and when connecting each of the semiconductor elements inparallel, the semiconductor elements in the power supply apparatus arearranged so as to be adjacent to each other.

[0045] Furthermore, a power supply apparatus according to the presentinvention, wherein the semiconductor element is a GaN-FET, in the abovedescribed invention.

[0046] According to the above-mentioned aspect of the present invention,the semiconductor element disposed in the path of a main current that isa subject of power control is formed of a GaN-FET, and the resistance atthe time of conduction is made extremely small. Thus, heat generated bythe semiconductor element is made little.

[0047] Furthermore, a power supply circuit according to the presentinvention having a transformer, and conducting on/off control on voltageapplied to a primary winding of the transformer, and thereby supplying astabilized power supply voltage to a secondary winding side of thetransformer, wherein the power supply circuit includes a GaN-FETconnected to the primary winding of the transformer andon/off-controlled by a gate signal.

[0048] According to the above-mentioned aspect of the present invention,the radiator having a large occupied area and a large weight is madeunnecessary by forming a transistor serving as a switching element byuse of a GaN-FET, which is small in generated heat.

[0049] Furthermore, a large current load control apparatus according tothe present invention that conducts on/off control on a current suppliedfrom a power source according to a predetermined instruction andsupplies a resultant current to an electric load, wherein the powersupply circuit includes a GaN-FET that is connected to a power sourceline for connecting the source to the load and that conducts on/offoperation according to the control.

[0050] According to the above-mentioned aspect of the present invention,the radiator which occupies a area and which is heavy is madeunnecessary by forming the on/off control switching element by use of aGaN-FET, which generates little heat and which can operate at hightemperature (at least 500°)

BRIEF DESCRIPTION OF THE DRAWINGS

[0051]FIG. 1 is a diagram showing a schematic circuit configuration of apower supply apparatus that is a first embodiment of the presentinvention; FIG. 2 is a diagram showing a configuration of GaN-FET shownin FIG. 1; FIG. 3(a) to (c) are diagrams showing differences instructure between a power supply apparatus using an FET of a Sisemiconductor and a power supply apparatus of a first embodiment using aGaN-FET; FIG. 4 is a diagram showing a schematic circuit configurationof a power supply apparatus that is a second embodiment of the presentinvention; FIG. 5(a) to (c) are diagrams showing differences instructure between a power supply apparatus using an FET of a Sisemiconductor and a power supply apparatus of a second embodiment usinga GaN-FET; FIG. 6 is a diagram showing a schematic circuit configurationof a power supply apparatus that is a third embodiment of the presentinvention; FIG. 7 is a diagram showing a schematic circuit configurationof another power supply apparatus which is a third embodiment of thepresent invention; FIG. 8(a) to (c) are diagrams showing a schematiccircuit configuration of another power supply apparatus that is a thirdembodiment of the present invention; FIG. 9 is a diagram showing aschematic circuit configuration of a power supply apparatus that is afourth embodiment of the present invention; FIG. 10 is a circuit diagramshowing an example of a configuration of a power supply circuitaccording to the present invention; FIG. 11 is a waveform diagramshowing the relation between the current of a coil L1 and on/offoperation of a GaN-FET 11; FIG. 12 is a waveform diagram showingcurrent-voltage waveforms of a primary side of a transformer shown inFIG. 10; FIG. 13 is a circuit diagram showing a circuit configuration ofa large current load control apparatus according to the presentinvention; FIG. 14 is a circuit diagram showing a circuit configurationof an overcurrent detection circuit shown in FIG. 13; FIG. 15 is acircuit diagram showing another circuit configuration of a large currentload control apparatus according to the present invention; and FIG. 16is an oblique view showing a configuration of a conventional powersupply apparatus.

BEST MODE FOR CARRYING OUT THE INVENTION

[0052] Embodiments of the power supply apparatus, power supply circuit,and the large current load control apparatus according to the presentinvention will be described in detail.

[0053]FIG. 1 is a diagram showing a schematic circuit configuration ofthe power supply apparatus according to a first embodiment of thepresent invention. This power supply apparatus is a linear regulator.This power supply apparatus is a stabilized power supply apparatus forconverting an input voltage Vin of 12 VDC to 5 VDC of maximum 10 A andoutputting the 5 VDC as an output voltage Vout.

[0054] In FIG. 1, a GaN-FET 10 is connected between an input terminal 11of an input voltage Vin side and an output terminal 12 of an outputvoltage Vout side. Drain D and source S of the GaN-FET 10 are connectedto the input terminal 11 side and the output terminal 12 side,respectively. Gate G of the GaN-FET 10 is connected to a Zener diode ZD.In other words, the GaN-FET 10 controls the main current, which flowsfrom the input voltage Vin side to the output terminal Vout side.

[0055] An electrolytic capacitor C1 is a capacitor for smoothing avoltage waveform in the case where the input voltage Vin is full-waverectified by a bridge diode and so on. The Zener diode ZD and a resistorR form a shunt regulator, and conducts voltage setting so as to convertthe input voltage of 12 V to the output voltage Vout of 5 V. Assumingnow that the terminal voltage of the Zener diode ZD is a voltage Vz andthe gate-source voltage of the GaN-FET 10 is a voltage Vgs, the outputvoltage Vout is represented by the following equation (1).

[0056] In other words, it follows that

Vout=Vz−Vgs   (1)

[0057] where the voltage Vz of the Zener diode ZD is concretely set to5.6 V and the voltage Vgs is 0.3 V. Therefore, the output voltage Voutis output as 5.6 V−0.3 V=5.3 V. A ceramic capacitor C2 is a capacitorfor preventing oscillation of the GaN-FET 10. Further more, anelectrolytic capacitor C3 is a capacitor for smoothing an instantaneousvariation of a load, which is not illustrated, connected to the outputvoltage Vout. As a result, the power supply apparatus shown in FIG. 1supplies a stabilized power supply voltage having an output voltage of5.3 V to the load.

[0058] As a general FET, a Si semiconductor, which can be easilysubjected to fine processing, is used. In the case of an FET that makesfast operation possible, a GaAs compound semiconductor is used. TheGaN-FET 10 is an FET that uses a GaN compound semiconductor and that hashigh heat-resisting property, high breakdown voltage, high operationrate and low conduction loss. The GaN-FET 10 has, for example, a HEMTtype or an MES (Metal-semiconductor) type FET structure.

[0059] In the GaN-FET 10 shown in FIG. 2, a GaN buffer layer 2 is formedon a semi-insulating sapphire substrate 1. On the GaN buffer layer 2, asemi-insulating GaN layer 3 is formed. On the semi-insulating GaN layer3, an n-type AlGaN layer 4 is formed. In addition, on a part of acentral portion of a surface layer portion of the n-type AlGaN layer 4,a diffusion layer 4 a with In and C or Mg doped is formed. On thediffusion layer 4 a, an electrode of the gate G is loaded. On theremaining portions of the surface layer portion of the n-type AlGaNlayer 4, an n-type GaN layer 5 is formed. Over one of the remainingportions of the surface layer portion of the n-type AlGaN layer 4, anelectrode of the source S is loaded. Over the other of the remainingportions of the surface layer portion of the n-type AlGaN layer 4, anelectrode of the drain D is loaded. Portions other than the electrodesare covered by an insulating film 6 of SiO.

[0060] Each of the semiconductor layers of the GaN-FET 10 shown in FIG.2 is formed of a GaN compound semiconductor, and formed by using anepitaxial crystal growth method such as the MOCVD method or the MBEmethod. The term GaN compound semiconductor is a general term of GaN,AlGaN, InGaN, InAlGaN, InGaNAs, InGaNP and so on.

[0061] In the GaN-FET 10, the on-resistance per unit area becomes nearly{fraction (1/100)} or less that of the FET of the Si semiconductor, andthe conduction loss is small. Therefore, the amount of heat generated bythe GaN-FET 10 becomes extremely small. Furthermore, the operationtemperature of the Si semiconductor is in the range of approximately125° C. to 150° C. at most, whereas the GaN-FET 10 can operate stablyeven at 500° C. A current of 10 A at most flows through the GaN-FET 10.Therefore, channel (junction) generated heat maximum temperature T1chmaxof the GaN-FET 10 will now be compared with channel generated heatmaximum temperature T2chmax of the FET of the Si semiconductor throughwhich a maximum current of 10 A flows, and studied.

[0062] By using a maximum ambient temperature Tamax, a maximumon-resistance Ronmax, a maximum on-current Ionmax, and a thermalresistance coefficient Rth(ch-a) between the channel and environment,the channel generated heat maximum temperature Tchmax can be representedby the following equation (2).

Tchmax=Tamax+Ronmax×Ionmax×Ionmax×Rth(ch-a)   (2)

[0063] Assuming that the maximum ambient temperature Tamax is 85° C.,the thermal resistance coefficient Rth(ch-a) is 50° C./W, and themaximum on-resistance Ronmax of the Si semiconductor is 0.013Ω, thechannel generated heat maximum temperature T2chmax of the FET of the Sisemiconductor becomes $\begin{matrix}{{{T2ch}\quad \max} = {85{^\circ}\quad {C.{+ 0.013}}\Omega \times 10A \times 10A \times 50{^\circ}\quad {{C.}/W}}} \\{= {150{^\circ}\quad {C.}}}\end{matrix}$

[0064] On the other hand, in the GaN-FET 10, the maximum on-resistanceRonmax is {fraction (1/100)} or less that of the FET of the Sisemiconductor. Therefore, the channel generated heat maximum temperatureT1chmax of the GaN-FET 10 becomes $\begin{matrix}{{{T2ch}\quad \max} = {85{^\circ}\quad {C.{+ \left( {0.013{\Omega/100}} \right)}} \times 10A \times 10A \times 50{^\circ}\quad {{C.}/W}}} \\{= {85.65{^\circ}\quad {C.}}}\end{matrix}$

[0065] When a maximum current of 10 A flows, therefore, the temperaturerises to 150° C. in the FET of the Si semiconductor. In the GaN-FET 10,however, the temperature is nearly the same as the ambient temperatureTamax, and there is little temperature rise. Therefore, a radiator forcooling the GaN-FET 10 of the power supply apparatus shown in FIG. 1becomes unnecessary.

[0066]FIG. 3 is a diagram showing differences in structure between thepower supply apparatus using the FET of the Si semiconductor and thepower supply apparatus using the GaN-FET 10. FIG. 3(a) is a sectional ofa power supply apparatus corresponding to the conventional power supplyapparatus shown in FIG. 10. In FIG. 3(a), a FET 20 of a Si semiconductoris used. The FET 20 generates much heat. Accordingly, a radiator 22 madeof aluminum having high conductivity is provided on the top of anapparatus main body 21. One end surface of the radiator 22 is opposed tothe apparatus main body 21, and it serves as a lid of the apparatus mainbody 21. On the other end surface of the radiator 22, radiation fins areprovided to radiate heat generated by the FET 20 to the environment.Most of the heat generated by the conventional power supply apparatus isoccupied by heat generated by the FET 20. Therefore, the FET 20 isjoined to the one end surface of the radiator 22 so as to make thecontact area large.

[0067] On the other hand, FIG. 3(b) is a sectional view of the powersupply apparatus using the GaN-FET 10. The power supply apparatus shownin FIG. 3(b) differs from the power supply apparatus shown in FIG. 3(a)in that the radiator 22 is not provided. As described above this isbecause the power supply apparatus shown in FIG. 3(b) uses the GaN-FET10, which generates less heat. In the power supply apparatus using theGaN-FET 10, therefore, the radiator 22, which is large in weight andvolume, can be eliminated. As a result, reduction of the size and weightof the power supply apparatus can be realized, and it becomesunnecessary to conduct the radiation design calculation for design ofthe radiator 22.

[0068] Furthermore, since the GaN-FET 10 itself does not generate heat,the GaN-FET 10 can be disposed in an arbitrary position of the apparatusmain body 23. Therefore, arrangement of components included in the powersupply apparatus, i.e., layout design can be conduct flexibly.

[0069] Furthermore, since heat generation of the GaN-FET 10 need not beconsidered, the radiation design of the whole power supply apparatus isfacilitated. In addition, since the layout design can be conductedflexibly, it becomes possible to integrate the layout of componentsincluded in the power supply apparatus as shown in FIG. 3(c). As aresult, it becomes possible to obtain a power supply apparatus 24contracted as compared with the power supply apparatus main body 23.Accordingly, further reduction of the size and weight of the powersupply apparatus is realized. Furthermore, because of reduced size andweight of the power supply apparatus and less heat generated by thepower supply apparatus, this power supply apparatus can be disposed inan arbitrary position of a device such as a vehicle using the powersupply apparatus.

[0070] The reason why the size of the GaN-FET 10 shown in FIG. 3(b) and(c) is reduced as compared with the size of the FET 20 shown in FIG.3(a) is that the amount of heat generated by the GaN-FET 10 is small andconsequently the radiation structure of the GaN-FET 10 itself becomesunnecessary and the size and weight of the GaN-FET 10 itself becomesmall.

[0071] The power supply apparatus shown in FIG. 1 is an example of thesimplest linear regulator. In addition, a circuit for stabilizing theoutput voltage Vout under the load variation may be provided. Forexample, the voltage Vz of the Zener diode ZD may be adjusted by usingresistors connected in series across the output voltage Vout, therebyconducting voltage division, using semifixed resistors as respectiveresistors, and adjusting the semifixed resistors finely.

[0072] Furthermore, there may be provided a protection circuit that usesa differential amplifier for comparing a voltage divided by resistorsconnected in series with a reference voltage and thereby conductsprotection against overcurrent and load shortcircuit. Since thetemperature of the GaN-FET 10 itself is hardly raised even by anovercurrent, however, the thermal protection circuit may be simplifiedor eliminated. In this case, other components can be prevented frombeing destroyed by providing a fuse or the like on the input voltage Vinside. As a result, in the power supply apparatus using the GaN-FET 10,the thermal protection circuit can be simplified or eliminated.Therefore, reduction in size and weight of the power supply apparatus isfurther promoted, and the time and labor required for the design of thepower supply apparatus can be reduced.

[0073] According to the first embodiment, the GaN-FET 10 having a lowon-resistance is used, and consequently the radiator 22 of the powersupply apparatus is not needed, and the GaN-FET 10 can be disposed in anarbitrary position in the power supply apparatus. Therefore, the powersupply apparatus can be remarkably reduced in size and weight.Furthermore, the time and labor required for the radiation designincluding the radiator and the design of the thermal protection circuitcan be reduced. In addition, the time and labor required for theradiation design including the radiator and the design of thermalprotection circuit can be reduced, and the GaN-FET 10 can be disposed inan arbitrary position in the power supply apparatus, therefore the timeand labor required for the layout design of the whole power supplyapparatus can also be reduced. In addition, since the amount of heatgenerated by the GaN-FET 10 itself is small and the GaN-FET 10 has aheat-resisting property of 500° C. or more, it becomes possible to usethe power supply apparatus for a long time and the maintenance requiredfor the power supply apparatus is also reduced.

[0074] A second embodiment of the present invention will now bedescribed. In a power supply apparatus according to the secondembodiment, the GaN-FETs 10 are connected in parallel.

[0075]FIG. 4 is a diagram showing a schematic circuit configuration of apower supply apparatus that is an embodiment of the present invention.In the power supply apparatus shown in FIG. 4, a GaN-FET 30 having thesame configuration as that of the GaN-FET 10 is connected in parallelwith the GaN-FET 10. The parallel connection of the GaN-FET 10 and theGaN-FET 30 unit connecting sources S, drains D and gates G of each ofthe GaN-FET 10 and together. Remaining configuration is the same as thatof the power supply apparatus shown in FIG. 1. The same components aredenoted by like characters.

[0076] As described above, each of the GaN-FETs 10 and 30 themselves hasan extremely small on-resistance. Therefore, the conduction loss is lowand the amount of generated heat is small. Therefore, adjacentarrangement of a plurality of GaN-FETs becomes possible. Parallelconnection of GaN-FETs that does not cause a large weight change and alarge volume change as compared with the case of the GaN-FET 10 alonebecomes possible. As a result, the maximum current value of the powersupply apparatus can be doubled. In other words, each of the GaN-FETs 10and 30 shown in FIG. 4 can flow a maximum current of 10 A. By connectingthem in parallel, however, a maximum current of 20 A can be flown. As aresult, the current supply capability of the whole power supplyapparatus can be doubled.

[0077]FIG. 5 is a diagram showing differences in structure between thepower supply apparatus using the FET of the Si semiconductor and thepower supply apparatus using the GaN-FETs 10 and 30 connected inparallel. FIG. 5(a) is a sectional of the conventional power supplyapparatus shown in FIG. 3(a). FIG. 5(b) is a sectional view of the powersupply apparatus having the GaN-FETs 10 and 30 connected in parallel andarranged so as to be adjacent to each other. In the case where a powersupply apparatus that flows a current of 20 A in the same way as theGaN-FETs 10 and 30 connected in parallel is implemented by using the FET20 of the Si semiconductor, the amount of heat generated by the FET 20becomes further large. Therefore, the radiator 22 shown in FIG. 5(a)must be made further larger.

[0078] On the other hand, although the apparatus shown in FIG. 5(b) hasa capability that is twice the amount of power supplied by the powersupply apparatus shown in FIG. 5(a), the radiator 22 is not needed andconsequently remarkable reduction in size and weight is realized.Furthermore, since the GaN-FETs 10 and 30 can be arranged so as to beadjacent to each other, design of the power supply apparatus is alsofacilitated.

[0079] In FIG. 5(c), a power supply apparatus further reduced in sizeand weight is implemented by conducting the layout design of respectivecomponents included in the power supply apparatus in the same way asFIG. 3(c).

[0080] By the way, the configuration having two GaN-FETs 10 and 30connected in parallel is shown In FIG. 4. However, the configuration isnot limited thereto, but a configuration having three or more GaN-FETsare connected in parallel may be used. In this case, the current supplycapability can be further improved.

[0081] In addition to the operation effects of the first embodiment,according to the second embodiment, the power supply capability of thepower supply apparatus itself can be doubled with the same weight,volume, and scale as those when one GaN-FET is used by only connectingthe GaN-FETs 10 and 30 in parallel. Furthermore, in view of the currentsituation that the development of a GaN-FET capable of flowing a largecurrent therethrough is under progress, the parallel connection ofGaN-FETs becomes effective unit which easily realizes the reduction ofsize and weight in large-power power supply apparatuses.

[0082] A third embodiment of the present invention will now bedescribed. In both the first and second embodiments, the power supplyapparatus serves as a linear regulator. In the third embodiment,however, the above described GaN-FET is used in a power supply apparatusserving as a switching regulator.

[0083]FIG. 6 is a diagram showing a schematic circuit configuration of apower supply apparatus that is a third embodiment of the presentinvention. The power supply apparatus shown in FIG. 6 is a switchingregulator. In other words, in the power supply apparatuses shown in thefirst and second embodiments the current value is controlled linearly byusing the GaN-FETs 10 and 30, whereas in this power supply apparatus thecurrent value is controlled by switching control.

[0084] The switching regulator shown in FIG. 6 is a switching regulatorof forward type. In this switching regulator, a pulse width modulation(PWM) signal output by a pulse width control circuit 41 is applied to aGaN-FET 40 at its gate, and the GaN-FET 40 is switched. When the GaN-FET40 is on, energy of an input voltage Vin stored across an electrolyticcapacitor C41 is transferred to an electrolytic capacitor C42 via atransformer T1, a diode D1 and an inductor L1. When the GaN-FET 40 isoff, energy left in the inductor L1 is transferred to the electrolyticcapacitor C42 via a diode D2. The electrolytic capacitor C42 outputs itas an output voltage Vout.

[0085] A differential amplifier 42 compares a voltage obtained byvoltage division using resistors R1 and R2 connected in series acrossthe output voltage Vout with a reference voltage Vref, and notifies thepulse width control circuit 41 of the control value caused by a loadvariation, via a photocoupler PC. The pulse width control circuit 41applies a PWM signal corresponding to the control value input from thephotocoupler PC to the GaN-FET 40 at its gate, controls the currentvalue of the GaN-FET 40, and thereby conducts power control of theoutput voltage Vout side (secondary side).

[0086] In this switching regulator, GaN-FET 40 is used as a switchingelement of a primary side. In the same way as the GaN-FET 10 and 30 inthe first and second embodiments, however, the GaN-FET 40 is smaller inon-resistance than the conventional FET of the Si semiconductor.Therefore, the amount of heat generated by the GaN-FET 40 itself islittle and the radiator for radiation becomes unnecessary.

[0087] Furthermore, since the heat generated by the GaN-FET 40 itself islittle and the radiator is unnecessary, the GaN-FET 40 can be disposedarbitrarily in the switching regulator. As a result, reduction of thesize and weight of the switching regulator can be realized, and inaddition the time and labor required for the design including theradiation design can be reduced.

[0088]FIG. 7 is a diagram showing a schematic circuit configuration ofanother power supply apparatus that is the third embodiment of thepresent invention. Although the power supply apparatus shown in FIG. 6is a switching regulator of forward type, the power supply apparatusshown in FIG. 7 is a switching regulator of flyback type. In otherwords, in the power supply apparatus shown in FIG. 6, power energy ofthe primary side is transferred to the secondary side when the GaN-FET40 is on. In the power supply apparatus shown in FIG. 7, power energy ofthe primary side is transferred to the secondary side when a GaN-FET 50is off.

[0089] With reference to FIG. 7, in this switching regulator, a pulsewidth modulation (PWM) signal output by a pulse width control circuit 51is applied to the GaN-FET 50 at its gate, and the GaN-FET 50 isswitched. A winding direction of a transformer T2 is different from awinding direction of a transformer T1. When the GaN-FET 50 is on, energyof an input voltage Vin is stored in the transformer T1. When theGaN-FET 50 is off, energy stored in the transformer T2 is transferred toan electrolytic capacitor C52 via a diode D3 and the electrolyticcapacitor C52 outputs an output voltage Vout.

[0090] A differential amplifier 52 compares a voltage obtained byvoltage division using resistors R1 and R2 connected in series acrossthe output voltage Vout with a reference voltage Vref, and notifies thepulse width control circuit 51 of the control value caused by a loadvariation, via a photocoupler PC. The pulse width control circuit 51applies a PWM signal corresponding to the control value input from thephotocoupler PC to the GaN-FET 50 at its gate, controls the currentvalue of the GaN-FET 50, and thereby conducts power control of theoutput voltage Vout side.

[0091] Since the switching regulator of flyback type also uses theGaN-FET 50, the radiator becomes unnecessary, reduction of the size andweight of the whole switching regulator is realized, and designincluding the radiation design can be conducted flexibly.

[0092] In the same way, FIG. 8 shows an example of another switchingregulator using a GaN-FET. FIG. 8(a) shows an example of a switchingregulator of push-pull type (center tap type). FIG. 8(b) shows anexample of a switching regulator of half bridge type. Furthermore, FIG.8(c) shows an example of a switching regulator of full bridge type. Inswitching regulators shown in FIG. 8(a) to 8(c), GaN-FETs 61, 62, 71, 72and 81 to 84 are used.

[0093] Since each of the switching regulators shown in FIG. 8(a) to 8(c)also uses the GaN-FETs 61, 62, 71, 72 and 81 to 84, the radiator becomesunnecessary, reduction of the size and weight of the whole switchingregulator is realized, and design including the radiation design can beconducted flexibly. In particular, since a plurality of switchingelements are used and a plurality of GaN-FETs 61, 62, 71, 72 and 81 to84 are used as each of the switching elements, adjacent arrangement ofGaN-FETs becomes possible and reduction of size and weight of theswitching regulator is promoted.

[0094] In other switching regulators as well, the above describedoperational effect can be achieved by using GaN-FETs as the switchingelements of the switching regulator in the same way. For example, theswitching regulator may be a self-excited switching regulator using aRCC (ringing choke coil) scheme.

[0095] All of the above described switching regulators use the pulsewidth control. However, switching regulators are not limited thereto.The current value of each GaN-FET may be controlled by the frequency ofpulses.

[0096] In the same way as the first and second embodiments, according tothe third embodiment, the radiator of the power supply apparatus is notrequired and GaN-FETs can be disposed in arbitrary positions in thepower supply apparatus even in the case where the power supply apparatusis a switching regulator, by using GaN-FETs, which are small inon-resistance, as the switching elements. Therefore, the power supplyapparatus can be remarkably reduced in size and weight.

[0097] Furthermore, the time and labor required for the radiation designincluding the radiator can be reduced. In addition, since the GaN-FETscan be disposed in arbitrary positions in the power supply apparatus,the time and labor required for the layout design of the whole powersupply apparatus can be reduced. In addition, since the amount of heatgenerated by the GaN-FETs themselves is small and the GaN-FETs have aheat-resisting property, it becomes possible to use the power supplyapparatus for a long time and the maintenance required for the powersupply apparatus is also reduced.

[0098] A fourth embodiment of the present invention will now bedescribed. All of the power supply apparatuses of the first to thirdembodiments are DC-DC converters. In the fourth embodiment, however,GaN-FETs are used as switching elements used in a DC-AC inverter.

[0099]FIG. 9 is a diagram showing a schematic circuit configuration of apower supply apparatus that is a fourth embodiment of the presentinvention. The power supply apparatus shown in FIG. 9 rectifies an ACcurrent supplied from a commercial three-phase AC power source 90, byusing a diode group included in a rectifying circuit 91, and smooths therectified current by using an electrolytic capacitor C91. The smoothedcurrent is converted to an AC current having a desired frequency and adesired output voltage by an inverter circuit 92. The AC current isoutput to an induction motor (IM) 94.

[0100] The inverter circuit 92 includes GaN-FET pairs 101 and 102, 103and 104, and 105 and 106 serving as switching element pairs respectivelycorresponding to the U phase, V phase and W phase. A drive controlsection 93 sends PWM signals corresponding to respective phases to theGaN-FET pairs 101 to 106, and conducts switching control on each of eachof the GaN-FET pairs 101 to 106, and three-phase AC power having adesired frequency and out put voltage is supplied. Gates of the GaN-FETS102, 104 and 106 are supplied with inverted signals of PWM signalssupplied to each of the GaN-FETs 101, 103 and 105.

[0101] Since the power supply apparatus serving as an inverter shown inFIG. 9 uses the GaN-FETs 101 to 106, the radiator becomes unnecessary,reduction of the size and weight of the whole switching regulator isrealized, and design including the radiation design can be conductedflexibly.

[0102] Even in the case of other power supply apparatuses serving asinverters, such as an inverter that is used in a rice cooking jar usinginduction heating and that converts a DC current to a desired AC currentby using one switching element, a similar operational effect can beobtained by using a GaN-FET as a switching element.

[0103] In the same way as the first to third embodiments, according tothe fourth embodiment, the radiator of the power supply apparatus is notrequired and GaN-FETs can be disposed in arbitrary positions in thepower supply apparatus, by using GaN-FETs, which are small inon-resistance, as the switching elements used in an inverter. Therefore,the power supply apparatus can be remarkably reduced in size and weight.Furthermore, the time and labor required for the radiation design fordesigning the radiator can be reduced. In addition, since the GaN-FETscan be disposed in arbitrary positions in the power supply apparatus,the time and labor required for the layout design of the whole powersupply apparatus can be reduced. In addition, since the amount of heatgenerated by the GaN-FETs themselves is small and the GaN-FETs have aheat-resisting property, it becomes possible to use the power supplyapparatus for a long time and the maintenance required for the powersupply apparatus is also reduced.

[0104] In the foregoing description of the first to fourth embodiments,FETs of MSE type are used as GaN-FETs. However, GaN-FETs are not limitedthereto, but they may be FETs of HEMT type or MOS type. Furthermore,various semiconductor elements such as thyristors, triacs, GTOthyristors, bipolar transistors, MOS-FETs, and IGBTs may also besemiconductor elements using GaN compound semiconductors.

[0105] Furthermore, in all of the first to fourth embodiments, there hasbeen explained the case where semiconductor elements using GaN compoundsemiconductors have been applied to the power supply apparatuses.However, the semiconductor elements are not limited thereto, but anysemiconductor elements using a semiconductor material capable of makingthe on-resistance small may be used. For example, semiconductor elementsusing a SiC compound semiconductor material or semiconductor elementsusing an AlN compound semiconductor material may be used.

[0106] An embodiment of a power supply circuit according to the presentinvention will now be described.

[0107] In FIG. 10, a power supply circuit is, for example, a switchingpower supply circuit (one-transistor forward type) . It includes atransformer T1 supplied with an input voltage Ein, a GaN-FET 200connected to a primary winding of the transformer T1, an electrolyticcapacitor C1 connected in parallel with the primary winding of thetransformer T1, a diode D1 and a coil L1 connected to a secondarywinding of the transformer T1, and a diode D2 and an electrolyticcapacitor C2 connected in parallel with the secondary winding of thetransformer T1. On the secondary winding side, a voltage E2 is generatedaccording to a winding ratio.

[0108] In the GaN-FET 200, for example, a GaN buffer layer 2 is formedon a semi-insulating sapphire substrate 1 as shown in Fig. 2. On the GaNbuffer layer 2, a semi-insulating GaN layer 3 and an n-type AlGaN layer4 are sequentially formed. In addition, on a part of a central portionof a surface layer portion of the n-type AlGaN layer 41 d, a diffusionlayer 4 a with In and C or Mg doped is formed. On the diffusion layer 4a, an electrode of the gate G is loaded.

[0109] Furthermore, on the remaining portions of the surface layerportion of the n-type AlGaN layer 4, an n-type GaN layer 5 is formed.Over the remaining portions of the surface layer portion of the n-typeAlGaN layer 4 and on one n-type GaN layer 5, an electrode of the sourceS is loaded. Over the other of the remaining portions of the surfacelayer portion of the n-type AlGaN layer 4 and on the other n-type GaNlayer 5, an electrode of the drain D is loaded. Portions other than theelectrodes of the gate G, the source S and the drain D are covered by aninsulating film 6 of SiO.

[0110] Each of the semiconductor layers of the GaN-FET 200 shown in FIG.2 is formed of a GaN compound semiconductor, and formed by using anepitaxial crystal growth method such as the MOCVD method or the MBEmethod. The term GaN compound semiconductor is a general term of GaN,AlGaN, InGaN, InAlGaN, InGaNAs, InGaNP and so on.

[0111] If a gate signal (for example, 100 kHz) is input to the gate ofthe GaN-FET 200, then the GaN-FET 200 turns on and off according to thegate signal. At this time, an input voltage Ein is supplied to a primarywinding of the transformer T1, and a voltage E2 is generated accordingto the winding ratio.

[0112] Assuming now that the ratio between the primary winding and thesecondary winding is N1:N2, the voltage E2 becomes

E 2=(N2/N1)×Ein

[0113] At this time, a voltage of a positive direction is supplied tothe diode D1, and consequently a current Is flows through the diode D1.Since this current Is charges the electrolytic capacitor C2 through thecoil L1, E0 is output as an output voltage. At the same time, energy isstored within the coil L1 by the current flown through the coil L1.

[0114] If the GaN-FET 200 passes through an ON period and turns off,then transmission of power from the primary winding side through thetransformer T1 disappears, and a voltage of an opposite polarity isgenerated in the coil L1. It is counter electromotive force caused bythe energy stored in the coil L1 until then. By this counterelectromotive force, such a current as to charge the electrolyticcapacitor C2 is flown through the diode D2. By the way, the electrolyticcapacitor C1 is a smoothing capacitor and functions so as to alwaysinput a flat voltage waveform to the transformer T1.

[0115] In this way, in the power supply circuit, the current thatcharges the capacitor C2 over the whole period continues to flow.

[0116] The gate of the GaN-FET 200 may be controlled by using anstabilizing circuit that monitors the load current, alters the on/offcontrol time of the GaN-FET 200 according to the load variation, andobtains a stabilized output.

[0117] Design of a circuit using the GaN-FET 200 will now be described.When conducting such a circuit design, it has here to fore beennecessary to conduct the radiation design of the FET accurately.Therefore, the design time becomes long, and it is necessary to considerthe layout of the print board and the like. The degree of freedom of thelayout is limited. In recent years, therefore, simplification andshortening of the radiation design of the FET have been desired.

[0118] On the other hand, in the present embodiment shown in FIG. 10, anoutput current of up to 30 A is obtained. Therefore, a current I t maxthat flows the transformer T1 is obtained by the following equation.

It max=(N2/N1)×Is max

[0119] Assuming now that the ratio of the transformer T1 is N1:N2=3:1and the ripple current is 30% of the output current I0, the current Ismax is

Is max=Io×1.15

[0120] Therefore, it is necessary to conduct on/off driving on a currentof

It max=(⅓)×30×1.15=11.5A

[0121] with the GaN-FET 200.

[0122] From a switching waveform shown in FIG. 12, the total loss Ptotalcan be derived by the following equations

Ptotal=Ps(on)+Pc+Ps(off)

Ps(on)=VDSmax×IL×tr×f/6

Pc=RDS(on)×(IL+Ip)²×Ton ×f/2

Ps(off)=Vp×Ip×tf×f/6

[0123] where

[0124] Ps(on): turn-on loss

[0125] Pc: conduction loss

[0126] Ps(off): turn-off loss

[0127] VDSmax: drain-source voltage

[0128] IL: minimum drain current

[0129] tr: turn-on time

[0130] f: frequency

[0131] RDS(on): on-resistance

[0132] Ip: maximum drain current

[0133] Ton: on-time

[0134] Vp: surge voltage

[0135] tf: turn-off time

[0136] For example, assuming that VDSmax=50 V, tr=tf=50 ns, f=100 kHs,RDS(on)=0.013/100, IL=10 A, Ip=11.5 A, Ton=4.9 μs, and Vp=60 V (see FIG.12), it follows that

[0137] Ps(on)=0.4W

[0138] Pc=0.01W

[0139] Ps(off)=0.57W

[0140] Therefore, the loss Ptotal becomes

Ptotal=0.4+0.01+0.57=0.98W

[0141] A channel temperature Tch max of the GaN-FET becomes$\begin{matrix}{{{Tch}\quad \max} = {{{Ta}\quad \max} + {{Ptotal} \times {{Rth}\left( {{ch} - a} \right)}}}} \\{= {85{^\circ}\quad {C.{+ 0.98}}W \times 50{^\circ}\quad {{C.}/W}}} \\{= {129{^\circ}\quad {C.}}}\end{matrix}$

[0142] Even if a power supply circuit for outputting 30 A is to beconstituted, it becomes possible to realize sufficient derating for thechannel temperature without a radiation plate, because a GaN-FET, whichgenerates less heat and which is capable of operating at hightemperature (stable operation at 500° C. or higher), is used as the FET.

[0143] Thus, in the present embodiment, a GaN-FET, which is small inon-resistance Ron max and which is capable of operating at hightemperature as compared with the conventional power MOS element, isused. As a result, the transistor does not generate heat, and the sameoperation as that of the conventional transistor can be conducted. Inaddition, the radiation plate becomes unnecessary. Accordingly, itbecomes possible to reduce the manufacturing cost, reduce the work costof the radiation plate, and reduce the size of the ECU.

[0144] Furthermore, in the present embodiment, the radiation design ofthe power supply circuit can be simplified and the circuit patterndesign becomes easy. As a result, the design time of the ECU can beshortened.

[0145] The present invention is not limited to these embodiments.Without departing from the spirit of the present invention, variousmodifications can be carried out. In the present embodiment,one-transistor forward type has been described as an example of aswitching power supply circuit. However, the present invention is notlimited thereto, but the present invention can also be applied to powersupply circuits of, for example, the chopper scheme, RCC scheme, andflyback scheme.

[0146] An embodiment of a large current load control apparatus accordingto the present invention will now be described.

[0147]FIG. 13 is a circuit diagram showing a circuit block of a largecurrent load control apparatus used in head lamp control in anautomobile. In the present invention, the circuit is formed by using aGaN-FET 211 instead of a power MOS-FET as an on/off control element ofhead lamps 210.

[0148] In the GaN-FET 211, for example, a GaN buffer layer 2 is formedon a semi-insulating sapphire substrate 1 as shown in FIG. 2. On the GaNbuffer layer 2, a semi-insulating GaN layer 3 and an n-type AlGaN layer4 are sequentially formed. In addition, on a part of a central portionof a surface layer portion of the n-type AlGaN layer 4, a diffusionlayer 4 a with In and C or Mg doped is formed. On the diffusion layer 4a, an electrode of the gate G is loaded.

[0149] Furthermore, on the remaining portions of the surface layerportion of the n-type AlGaN layer 4, an n-type GaN layer 5 is formed.Over the remaining portions of the surface layer portion of the n-typeAlGaN layer 4 and on one n-type GaN layer 5, an electrode of the sourceS is loaded. Over the other of the remaining portions of the surfacelayer portion of the n-type AlGaN layer 4 and on the other n-type GaNlayer 5, an electrode of the drain D is loaded. Portions other than eachof the electrodes of the gate G, the source S and the drain D arecovered by an insulating film 6 of SiO.

[0150] Each of the semiconductor layers of the GaN-FET 211 shown in FIG.2 is formed of a GaN compound semiconductor, and formed by using anepitaxial crystal growth method such as the MOCVD method or the MBEmethod. The term GaN compound semiconductor is a general term of GaN,AlGaN, InGaN, InAlGaN, InGaNAs, InGaNP and so on.

[0151] In the present embodiment, FIG. 13 shows a high-side drivecircuit formed by connecting the GaN-FET 211 to a power source line 201between a battery serving as an internal power source and the head lamps210 serving as electric loads. The drain of the GaN-FET 211 is connectedto the battery. The source is connected to two head lamps 210. Aresistor R1 and a capacitor C1 are connected to the gate of the GaN-FET211. In addition, a microcomputer 214 serving as a control circuit isalso connected to the gate of the GaN-FET 211 via a FET 212 and a FET213. Under the control of the microcomputer 214, the GaN-FET 211conducts on/off operation. Furthermore, between the gate and the source,a diode D1, a Zener diode D2 and a resistor R2 are connected in series.

[0152] A charge pump circuit 215 is connected to the FET 212 at itssource. Between the source and gate of the FET 212, a resistor R3 isconnected to raise the voltage supplied to the FET 212. A resistor R4 isconnected to the gate of the FET 212. Resistors R5 and R6 and acapacitor C2 are connected to the gate of the FET 212. In addition, themicrocomputer 214 is also connected to the gate of the FET 212.

[0153] The microcomputer 214 is connected to the battery via a powersupply circuit 216. The power supply circuit 216 performs conversion onthe power supply voltage supplied from the battery, and supplies aresultant voltage to the microcomputer 214. A switch 217 for conductingon/off switchover of the head lamps 210 is connected to themicrocomputer 214. In the present embodiment, the switch 217 for on/offswitchover is used. However, on/off control maybe conducted by using aCAN (control area network), which is an intra-vehicle LAN (local areanetwork), or the like.

[0154] If the switch 217 assumes the on-state in the above describedlarge current load control apparatus, then the microcomputer 214 sensesthe on-state from an input port connected to the switch 217, and outputsa signal of a high level (5 V) to an output port for controlling thehead lamps 210. By this output, the FETs 212 and 213 are turned on. TheGaN-FET 211 is controlled so as to turn on, and the head lamps 210 arelit. In the present embodiment, the GaN-FET 211 is located on theupstream side (battery side) of the head lamps 210. Because of such highside drive, there is the charge pump circuit 215 on the source side ofthe FET 212. This charge pump circuit 215 is set so as to input avoltage that is equal to at least the sum of the battery voltage and thegate-source voltage of the GaN-FET 211 to the gate of the GaN-FET 211 inorder to turn on the GaN-FET 211. The charge pump circuit 215 of thepresent embodiment is set so as to, for example, convert the batteryvoltage to 21 V and supply the 21 V to the gate of the GaN-FET 211.

[0155] Furthermore, if the switch 217 turns off, the microcomputer 214senses the off-state, and outputs a low level (0 V) to the output portfor controlling the head lamps 210. By this output, the FETs 212 and 213are turned off. The GaN-FET 211 is controlled so as to turn off, and thehead lamps 210 are put out.

[0156] Furthermore, in the present embodiment, a shunt resistor R7 isconnected between the GaN-FET 211 and the battery, and an overcurrentdetection circuit 218 is connected across the shunt resistor R7 todetect an overcurrent that flows through the GaN-FET 211. As shown inFIG. 3, the overcurrent detection circuit 218 includes two operationalamplifiers 219 and 220. The overcurrent detection circuit 218 amplifiesand detects a current that flows through the shunt resistor R7, andoutputs a result of detection to the microcomputer 214. If an excessivecurrent continuously flows, then the wire harness is heated and there isa possibility of degradation and smoke emitting. On the basis of aresult of the detection, the microcomputer 214 exercises control so asto turn off the GaN-FET 211.

[0157] Design of a circuit using the GaN-FET 211 will now be described.When conducting such a circuit design, it has heretofore been necessaryto conduct the radiation design of the FET accurately. Therefore, thedesign time becomes long, and it is necessary to consider the layout ofthe print board and the like. The degree of freedom of the layout islimited. In recent years, therefore, simplification and shortening ofthe radiation design of the FET have been desired.

[0158] On the other hand, in the present embodiment shown in FIG. 13,power of 60 W×2=120 W is required to turn on two head lamps 210. In asteady state, a maximum current of approximately 10 A flows. Fromequation (10), therefore, the channel temperature of the GaN-FETs 211 atan ambient temperature of 85° C. is calculated as $\begin{matrix}{{{Tch}\quad \max} = {85{^\circ}\quad {C.{+ \left( {0.013{\Omega/100}} \right)}} \times 10A \times 10A \times 50{^\circ}\quad {{C.}/W}}} \\{= {85.65{^\circ}\quad {C.}}}\end{matrix}$

[0159] Even if a current of 10 A is always flown, heat is generated atall. Accordingly, the radiation plate required when the power MOS-FETsare used becomes unnecessary.

[0160] Furthermore, in the case where a large current load controlapparatus is used in a severe temperature environment such as an engineroom, the use temperature range of the apparatus is required to be atleast 125°. By using GaN-FETs capable of operating at high temperature(operating stably even at 500°) different from the conventionalMOS-FETs, it is possible to set sufficient derating (at least 500°) withrespect to the channel temperature, and design of a highly reliable,small-sized ECU is facilitated.

[0161] Thus, in the present embodiment, GaN-FETs, which are small inon-resistance Ron max as compared with the conventional power MOSelements, are used. As a result, on/off control elements do not generateheat, and the same operation as that of the conventional on/off controlelements can be conducted. In addition, the radiation plate becomesunnecessary. Accordingly, it becomes possible to reduce themanufacturing cost, reduce the work cost of the radiation plate, andreduce the size of the ECU.

[0162] Furthermore, in the present embodiment, the radiation design ofthe circuit can be simplified and the circuit pattern design becomeseasy. As a result, the design time of the ECU can be shortened.

[0163] The present invention is not limited to these embodiments.Without departing from the spirit of the present invention, variousmodifications can be carried out. In the present embodiment, a suitableexample of a high-side type head lamp control circuit for automobile hasbeen described. However, the present invention is not limited thereto,but it is also possible to adopt, for example, a low-side drive circuitconfiguration having a GaN-FET 211 connected between a head lamp 210 andGND as shown in FIG. 15. In FIG. 15, a control unit 220 is obtained byincorporating elements, such as a microcomputer and the FET forconducting on/off control on the GaN-FET 211, in a unit.

[0164] Furthermore, as for the GaN-FET, any of a GaN-FET of N-channeltype and a GaN-FET of P-channel may be used.

[0165] Furthermore, a large current load control apparatus according tothe present invention may be used for control of, for example, taillamps or fog lamps, other than head lamps. Or a large current loadcontrol apparatus according to the present invention may have functionsof these lamps controls together. In addition, it is also possible touse a large current load control apparatus according to the presentinvention for motor control, for example on/off control of a load, suchas a floor motor or wiper motor (HI, LOW, INT, MIST) for automobiles.

[0166] As heretofore described, according to a power supply apparatusaccording to the present invention, a semiconductor element formed byusing a GaN compound is disposed in the path of a main current that is asubject of power control, and control unit controls conduction of themain current flowing through the semiconductor element. Since thesemiconductor element is small in resistance at the time of conduction,little heat is generated and it becomes unnecessary to provide the powersupply apparatus with a radiator having a large weight and a largevolume. This brings about an effect that reduction of the power supplyapparatus in size and weight can be realized and the time and laborrequired for radiation design can be remarkably reduced. Furthermore, itbecomes unnecessary to make the semiconductor element stick to aradiator. The semiconductor element can be disposed in an arbitraryposition in the power supply apparatus. This brings about an- effectthat the degree of freedom in the design of the power supply apparatusis increased in addition to easiness of the radiation design, eventuallymaking possible integration of elements arranged in the power supplyapparatus, and reduction of the power supply apparatus in size andweight is further promoted. In addition, there is brought about aneffect that the thermal runaway is eliminated and consequently a thermalprotection circuit such as an overcurrent protection circuit can besimplified.

[0167] Furthermore, according to a power supply apparatus according tothe present invention, a semiconductor element formed by using a GaNcompound disposed in the path of a main current that is a subject ofpower control, and control unit conducts switching control on conductionof the main current flowing through the semiconductor element. Since thesemiconductor element is small in resistance at the time of conduction,little heat is generated and it becomes unnecessary to provide the powersupply apparatus with a radiator having a large weight and a largevolume. This brings about an effect that reduction of the power supplyapparatus in size and weight can be realized and the time and laborrequired for radiation design can be remarkably reduced. Furthermore, itbecomes unnecessary to make the semiconductor element stick to aradiator. The semiconductor element can be disposed in an arbitraryposition in the power supply apparatus. This brings about an effect thatthe degree of freedom in the design of the power supply apparatus isincreased in addition to easiness of the radiation design, eventuallymaking possible integration of elements arranged in the power supplyapparatus, and reduction of the power supply apparatus in size andweight is further promoted. In addition, there is brought about aneffect that the thermal runaway is eliminated and consequently a thermalprotection circuit such as an overcurrent protection circuit can besimplified.

[0168] Furthermore, according to a power supply apparatus according tothe present invention, the power supply apparatus includes a pluralityof the semiconductor elements disposed in the path of a main currentthat is a subject of power control, and the plurality of semiconductorelements are connected in parallel. The limit of the main current thatcan be controlled is remarkably improved. In addition, the semiconductorelements generate little heat. This brings about an effect that a powersupply apparatus having high power control capability can be implementedby using a power supply apparatus having nearly the same weight andvolume as those of a power supply apparatus equipped with onesemiconductor element.

[0169] Furthermore, according to a power supply apparatus according tothe present invention, the power supply apparatus includes a pluralityof semiconductor elements disposed in the path of a main current that isa subject of power control, and when connecting the semiconductorelements in parallel, the semiconductor elements in the power supplyapparatus are arranged so as to be adjacent to each other, because thesemiconductor elements themselves generate little heat. This bringsabout an effect that the degree of freedom of design can be furtherimproved.

[0170] Furthermore, according to a power supply apparatus according tothe present invention, the semiconductor element disposed in the path ofa main current that is a subject of power control is formed of aGaN-FET, and the resistance at the time of conduction is made extremelysmall. Thus, heat generated by the semiconductor element is made little.This brings about an effect that reduction of the power supply apparatusin size and weight is further promoted and the time and labor requiredfor design including the radiation design can be remarkably decreased.

[0171] Furthermore, according to a power supply circuit according to thepresent invention, a GaN-FET, which is small in generated heat, is usedas a switching element of the power supply circuit. This brings about aneffect that reduced heat generation of the switching element makes theradiation plate unnecessary and the power supply circuit can be reducedin size and weight.

[0172] Furthermore, according to a large current load control apparatusaccording to the present invention, a GaN-FET, which is small ingenerated heat and which can operate at high temperature (at least 500°C.), is used as an on/off control switching element of the large currentload control apparatus. This brings about an effect that reduced heatgeneration of the switching element makes the radiation plateunnecessary and the power supply circuit can be reduced in size andweight.

INDUSTRIAL APPLICABILITY

[0173] As heretofore described, the power supply apparatus, power supplycircuit, and the large current load control apparatus are useful forautomobiles, electric vehicles, construction machinery, various publicwelfare devices (such as video devices, television sets, and audiodevices), various industrial devices (such as personal computers,communication devices, and FA control devices), and so on. They aresuitable for realizing reduction in size and weight of an apparatus orcircuit.

1. A power supply apparatus comprising: a semiconductor element,disposed in the path of a main current that is a subject of powercontrol, and formed by using a GaN compound; and a control unit whichcontrols conduction of the main current flowing through saidsemiconductor element.
 2. The power supply apparatus according to claim1, wherein said semiconductor element is provided in plurality, and saidsemiconductor elements are connected in parallel.
 3. The power supplyapparatus according to claim 2, wherein said semiconductor elements arearranged so as to be adjacent to each other.
 4. The power supplyapparatus according to claim 1, wherein said semiconductor element is aGaN-FET.
 5. A power supply apparatus comprising: a semiconductorelement, disposed in the path of a main current that is a subject ofpower control, and formed by using a GaN compound; and a control unitwhich conducts switching control on conduction of the main currentflowing through said semiconductor element.
 6. The power supplyapparatus according to claim 5, where in said semiconductor element isprovided in plurality, and said semiconductor elements are connected inparallel.
 7. The power supply apparatus according to claim 6, whereinsaid semiconductor elements are arranged so as to be adjacent to eachother.
 8. The power supply apparatus according to claim 5, wherein saidsemiconductor element is a GaN-FET.
 9. A power supply circuit having atransformer, and which conducts on/off control on voltage applied to aprimary winding of said transformer, and thereby supplies a stabilizedpower supply voltage to a secondary winding side of said transformer,further comprising: a GaN-FET connected to the primary winding of saidtransformer and on/off-controlled by a gate signal.
 10. A large currentload current load control apparatus that conducts on/off control on acurrent supplied from a power source according to a predeterminedinstruction and supplies a resultant current to an electric load, saidlarge current load current load control apparatus comprising: a GaN-FET,connected to a power source line that connects said source to said load,and which conducts the on/off control.